1. Field of the Invention
The present invention relates to a liquid jet recording head for recording onto a recording medium by discharging liquid (ink) droplets through discharge orifices, and particularly to a method for fabricating the liquid jet recording head having tapered electrode ends.
2. Related Background Art
Of a variety of well known recording methods at present, a so-called liquid jet recording method (ink jet recording method) is significantly effective because it is a non-impact recording method which produces less noise during the recording, has the high speed of recording, and requires no special fixing treatment on ordinary papers for recording.
The liquid jet recording method relies on jetting fine droplets of recording liquid referred to as the ink with various operation principles to attach onto the recording medium such as a paper, and its basic principle has been proposed in Japanese Laid-open Patent Application No. 52-118798 and will be outlined below. That is, this liquid jet recording method is one in which heat pulses as information signal are applied to the recording liquid within an operation chamber having recording liquid introduced therein, and in accordance with an operation force produced by the recording liquid developing into vapor bubbles, the recording liquid is discharged through discharge orifices communicating to the operation chamber, so that fine liquid droplets are jetted and then fixed onto the recording medium.
By the way, in forming heat energy generating means of a liquid jet recording head for use with such liquid jet recording method, it is common practice that a heat generating resistive layer is formed on a desired substrate, and then electrodes and a protective layer are laminated sequentially, wherein it is required that such protective layer of heat energy generating means may cover uniformly necessary portions of the heat generating resistive layer as well as the electrodes, without having any defects such as pinholes, to be able to sufficiently perform various functions as the protective layer, including prevention of breakage of heat generating resistive layer or short-circuit between electrodes.
As previously described, however, the electrodes are typically formed on the heat generating resistive layer, so that a step is produced between the electrodes and the heat generating resistive layer, but this step may yield uneven layer thickness. Hence, it is necessary, on this step portion, that layer formation enough to cover the step (step coverage) must be carried out to prevent any exposed portions from arising due to defects such as pinholes. That is, there were cases where in the insufficient step coverage state, the exposed portion of the heat generating resistive layer may be placed into direct contact with the recording liquid to electrically decompose the recording liquid or break the heat generating resistive layer due to reaction of material of the heat generating resistive layer with the recording liquid. Also, in such step portion, uneven film quality may likely occur, and possibly bring about partial concentration of heat stress developed in the protective layer due to repetitive heat generation, causing cracks on the protective layer, in which there were cases where recording liquid might penetrate through cracks to lead to breakage of the heat generating resistive layer as mentioned above. Further, the recording liquid may enter through pinholes to break down the heat generating resistive layer.
Conventionally, to resolve such a problem, it is common practice to increase the thickness of the protective layer to improve the step coverage or reduce pinholes. However, the greater thickness of the protective layer may contribute to the step coverage or reduction of pinholes, but may impede the heat supply to the recording liquid, resulting in a new problem as described below.
That is, the heat generated in the heat generating resistive layer will transfer through the protective layer to the recording liquid, wherein the heat resistance between the protective layer to apply the heat and the heat generating resistive layer will increase with greater thickness of the protective layer, requiring more electric load than necessary to be applied on the heat generating resistive layer, so that the following problems may be encountered:
1. Inefficient power saving.
2. More heat than necessary stored in substrate, with poorer heat responsibility.
3. Reduced durability of heat generating resistive layer due to greater power supply than necessary.
Such problems can be overcome by reducing the thickness of protective layer, but in a conventional fabrication method for liquid jet recording heads which uses the film formation method such as sputtering or vapor deposition for forming the protective layer, there was a drawback in the respect of durability as previously described, due to false step coverage, so that it was difficult to reduce the thickness of the protective layer.
In recording with the liquid jet recording head as above described, it is typically well known that the stability in bubbling the recording liquid can be improved by rapid heating of the recording liquid. That is, the electric signal to be applied to heat energy generating means, typically in the form of rectangular suspend pulses, has a property that the shorter the pulse width, the more excellent the bubbling stability of recording liquid, which can improve discharge stability of fine liquid droplets and recording qualities. However, the conventional liquid jet recording head is required to increase the layer thickness of the protective layer as previously described, which will also increase the heat resistance of the protective layer, thereby giving rise to a need of generating more heat than necessary in heat energy generating means, resulting in less durability and reduced heat responsibility. Therefore, it is difficult to shorten the pulse width, essentially resulting in a limitation in improving the recording quality.
Accordingly, a propose has been made in which the film thickness of a protective layer is reduced without any adverse effects on step coverage. For example, Japanese Laid-open Patent Application No. 60-234850 has proposed a bias sputter exhibiting excellent step coverage as a film forming method of the protective layer, Japanese Laid-open Patent Application Nos. 62-45283 and 62-45237 have proposed to improve step coverage by changing the shape of step by etchback or sputter etch after forming the protective layer, and Japanese Laid-Open Patent Application No. 62-45286 has proposed to improve step coverage by reflowing the protective layer.
However, the bias sputter has a drawback that the stability of film thickness is inferior and dirts may be produced around target. Also, the methods of etchback, sputter etch, and reflow may increase the number of processes, thereby raising up the costs.
Other methods include making a tapered cross-sectional shape of electrode to improve the step coverage of protective film, as proposed in HP journal, May, 1985. Also, it has been proposed to make a tapered shape of electrode by using a developer which is an alkali solution, and etching the resist together with electrodes (U.S. patent application No. 871,188, filed on Apr. 20, 1992 to the common assignee). The tapered cross-sectional shape of electrode is significantly effective means to improve the step coverage without increasing the film thickness of protective film, wherein its effects may greatly change with taper shape.
That is, when the taper is steep, the step coverage is insufficient, giving rise to a previously described problem. On the other hand, when the taper is gentle, the electrode wiring is reduced in width and cross section to produce higher resistance than other portions to have a dispersion of wiring resistance within the liquid jet recording head, resulting in a problem that the print quality of recording apparatus may be adversely affected.
In order to control the taper shape, the above-described method controls the concentration and temperature of developer, but due to instability of the etching rate for electrode and resist, there occurred some dispersion in taper shape, particularly when the taper shape became gentle. This dispersion of taper shape has led to dispersion of resistance value, whereby the print quality may degrade particularly for larger substrates.